Add BSWAP4/BSWAP8 routines for riscv64 with Zbb
Add BSWAP4/BSWAP8 routines for riscv64 with Zbb

These routines make use of the rev8 instruction in the Zbb extension
to accelerate byte-swapping when OpenSSL is built specifically for
a machine that supports Zbb.

Reviewed-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Signed-off-by: Henry Brausen <henry.brausen@vrull.eu>

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17640)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl